Under the Resource Conservation and Recovery Act (RCRA), the US Environmental Protection Agency (EPA) is required to set levels of treatment which substantially reduce the toxicity of hazardous wastes or substantially reduce the likelihood of migration of hazardous constituents from wastes. The current treatment standard for wastes exhibiting the toxicity characteristic for selenium is based upon the performance of stabilization treatment technology. EPA's performance criterion for the stabilization of a selenium characteristically hazardous waste was developed in 1990 (Rosengrant et al., EPA 530-SW-90-059A, May, 1990) using a mineral processing waste determined at that time to be the most difficult to treat selenium waste. The untreated waste upon which the criterion was based contained only 700 ppm (0.07%) total selenium and yielded an untreated TCLP (toxicity characteristic leaching procedure) leachate concentration of 3.74 mg/L. The resulting post-treatment level of selenium led to establishment of a national treatment TCLP standard of 5.7 mg/L for selenium-bearing wastes.
It subsequently became apparent to the EPA that selenium bearing wastes generated by the glass manufacturing industry frequently contained selenium concentrations much higher than those EPA had examined when it established the national treatment standard for selenium. In some cases, for example, the total selenium concentrations range from 10,000 ppm to over 80,000 ppm (or 1% to over 8%). Based upon the high levels of selenium in glass manufacturing wastes and the inability to treat the wastes to be in compliance with the national treatment TCLP standard of 5.7 mg/L for selenium-bearing wastes, petitions were submitted to the EPA requesting site-specific variances for stabilization of selenium wastes from glass manufactures.
Based on test results for alternative stabilization methods submitted by an initial petitioner, the EPA concluded that the best demonstrated available technology (referred to as BDAT-1) for stabilizing the selenium wastes involved combining wastes with a mix of cement and ferrous sulfate (FR Oct. 23, 1998). Typical reagent-to-waste ratios for this treatment protocol were 2.0:1 cement and 0.7:1 ferrous sulfate resulting in an overall reagent-to-waste ratio of 2.7 to 1. The addition of ferrous sulfate was primarily for the purpose of serving as a reducing reagent for stabilizing hexavalent chrome when present in the waste material.
Supplemental testing results submitted by a second petitioner (FR Feb. 11, 2004) enabled the EPA to establish an improved BDAT stabilization method for high-selenium wastes which involved adding a mix of cement, cement kiln dust and ferrous sulfate to waste materials. Typical dose rates for this improved technology were 1.0 part cement, 1.0 part cement kiln dust and 0.35 parts ferrous sulfate heptahydrate per part of waste with an overall reagent-to-waste ratio of 2.35.
When using these formulations water is typically added to make a thick paste that upon curing solidifies the treated material into a hard cementitious material. The EPA granted site specific variances for application of these technologies to selenium bearing wastes from three glass manufactures (FR Feb. 11, 2004). Depending on the facility, the variances ranged from TLCP selenium concentrations of 25 mg/l to 51 mg/l.
Several technologies are available for removing multivalent oxyanions from water or wastewater. Perhaps the most widely used method is co-precipitation with ferric hydroxide or aluminum hydroxide as discussed in the textbook “Water Treatment Principles & Design” (James M. Montgomery, Consulting Engineers, Inc., John Wiley & Sons, New York (1985)). Iron co-precipitation and aluminum co-precipitation methods are generally most effective at pHs of 7 or lower and are sensitive to the valence of the particular multivalent oxyanion. Iron or aluminum co-precipitation is effective for arsenate and selenite, but not for arsenite and selenate.
A number of patents are directed to “cementation” techniques that involve chemical reduction of specific oxyanions to their insoluble elemental form by oxidation of a metal that is added in its metallic form. For example, U.S. Pat. No. 3,933,635 to Marchant discloses the use of metallic zinc, iron or aluminum at an acidic pH to chemically reduce selenium ions to elemental selenium. The method can be used to remove selenium from zinc smelter acidic wastewater by treating the wastewater with metallic zinc.
Certain bacteria can also be used to chemically reduce multivalent oxyanions. For example, U.S. Pat. No. 4,519,913 to Baldwin et al. discloses the use of an anaerobic bacteria of the genus Clostridium for the biochemical reduction of selenium ions to insoluble metallic selenium.
U.S. Pat. No. 4,806,264 to Murphy discloses another technique in which ferrous iron is added to a wastewater at a pH of about 9 to chemically reduce selenate and selenite to elemental selenium.
Removal of multivalent oxyanions by applications of ion exchange and electrowinning technologies have also been developed as exemplified in U.S. Pat. No. 5,453,201 to Etzel et al. and U.S. Pat. No. 5,322,600 to Spitz et al.
Several patents are directed to techniques that remove oxyanions from water using activated or calcined hydrotalcite. For example, U.S. Pat. No. 4,458,030 to Manabe discloses the use of calcined magnesium or zinc hydrotalcite blended with activated carbon to effect an apparent synergistic effect for removing specific organic compounds and chromate and arsenate. U.S. Pat. No. 4,752,396 to Sood discloses an adsorbent for metal removal that consists of from 20 to 100 wt. % activated hydrotalcite and a balance of activated alumina. U.S. Pat. No. 4,935,146 to O'neill et al. discloses the use of calcined hydrotalcite at alkaline pHs to remove selenate, selenite and arsenite from water or wastewater.
It was concluded from laboratory testing and/or cost considerations that none of these water and wastewater treatment techniques were workable and/or practical for adaptation to the stabilization of residues containing multivalent oxyanions. Moreover, because stabilization with portland cement was moderately effective and had already been cited by the EPA as the best demonstrated technology for stabilizing selenium bearing residues generated by several glass manufacturers, subsequent research was directed towards enhancing the performance of portland cement as a stabilization reagent by the addition of supplemental reagents while maintaining an alkaline, pH.
Klemm (“Ettringite and Oxyanion-Substituted Ettringites”, Research and Development Bulletin RD116W, Portland Cement Association, 1998) provides an in-depth review and analysis of published literature as it relates to the stabilization of multivalent oxyanions through the formation of ettringite. Klemm's objective was to provide a foundation of knowledge upon which portland cement can be adapted to optimize ettringite's metals fixation characteristics. Some of the key conclusions were as follows.
The designation ettringite is most commonly thought of in terms of the calcium trisulfoaluminate hydrate, designated as:Ca6Al2(OH)12(SO4)3.26H2O
During the initial stage of portland cement hydration, tricalcium aluminate reacts with calcium sulfate (gypsum) to immediately form ettringite:Ca3Al2O6+3CaSO4.2H2O+26H2O→Ca6Al2(OH)12(SO4)3.26H2O  (1)
However, the limited amount of gypsum relative to the amount of aluminate results in a second step reaction in which much of the ettringite is converted into calcium monosulfoaluminate:Ca6Al2(OH)12(SO4)3.26H2O+2Ca3Al2O6+4H2O→3Ca4Al2(OH)12(SO4).6H2O  (2)
Ettringite has a much lower solubility than calcium monosulfate and thus appears to be most stable.
The composition of the ettringite that forms in cement is very different from the pure mineral phase. For example, calcium can be substituted by strontium and aluminum can be substituted by other trivalent metals such as trivalent chrome. Oxyanion species including selenate, selenite, borate, sulfite, arsenate and chromate can be incorporated into the structure of ettringite at high pH levels by substitution for sulfate in the ettringite.
A major advantage of ettringite-based chemistry, as applied to the stabilization of residues, is the extremely low solubility of ettringite at alkaline pHs in the range of 10.4 to 13.7, thereby providing a strong resistance to leaching.
It has long been recognized that ettringite can be easily prepared from mixtures of lime and aluminum sulfate, and that the reaction occurs very quickly, generally within an hour.
Tests were reported by others that showed that significant ettringite can form in pastes made from various high-calcium coal combustion fly ashes and residues which showed a positive correlation between the amount of ettringite formed and the resistance to selenium and boron leaching.
Zhang et al. (“Removal of B, Cr, Mo, and Se from Wastewater by Incorporation into Hydrocalumite and Ettringite,” Environ. Sci. Technol., 2003, Vol. 37, No. 13, pp 2947–2952) reported on the removal of various multivalent oxyanions from wastewater by incorporation into ettringite or hydrocalumite. The tests were performed by directly precipitating ettringite or hydrocalumite from solutions containing selenate, borate, chromate and molybdate. The ettringite was precipitated by adding hydrated lime and aluminum sulfate in accordance with the following reaction:6Ca(OH)2+Al2(SO4)3+26H2O→Ca6Al2(OH)12(SO4)3.26H2O  (3)
In a separate test, hydrocalumite was precipitated by adding hydrated lime and monocalcium aluminate in accordance with the following reaction:3Ca(OH)2+CaAl2O4+10H2O→Ca4Al2(OH)12(OH)2.6H2O  (4)
Excess lime was added in both cases to ensure the solutions were highly alkaline. X-ray diffraction (XRD) testing was performed to verify that the principal solid phases were indeed ettringite and hydrocalumite for the respective tests. The XRD patterns showed that ettringite formation was nearly complete after only 1 day of reaction, whereas formation of hydrocalumite took between 7 and 30 days to fully develop. The tests indicated that ettringite and hydrocalumite are both capable of greatly reducing the concentrations of borate, chromate, molybdate and selenate from solutions. Removals by ettringite showed a preference in the order of borate>selenate>chromate>molybdate with equilibrium concentrations (at pH around 12.5) of about <0.08 mg/L B, 2 mg/L Se, 4 mg/L Cr and 5 mg/L Mo. A review of the technical literature indicated that direct substitution for sulfate is likely the dominant mechanism for uptake of oxyanions by ettringite. The investigators pointed out that both ettringite and hydrocalumite are unstable at low pH and that pH values greater than 10.7 are required for ettringite and pH values greater than 11.6 are required for hydrocalumite.
Bauer et al. (“Sorption of Selenite and Selenate to Cement Materials,” Environ. Sci. Technol., 2003, Vol. 37, No. 15, pp 3442–3447) recently conducted experiments on the sorption of selenite and selenate to ettringite solids (calcium trisulfoaluminate) and “monosulfate” (calcium monosulfoaluminate). The protocol was considerably different from that used by Zhang et al. in that preformed ettringite and monosulfate solids were used that had equilibrated for over seven days before contacting the selenite and selenate. These results suggest that substitution of selenium oxyanions for sulfate is more difficult with preformed ettringite or monosulfate.
Although the previously described technologies acknowledged by the EPA for selenium stabilization are currently viewed as the best available, the TCLP selenium concentrations that could be reliably achieved by them remain much higher than the original 5.7 mg/L TCLP criterion. The present invention provides a superior stabilization technology for selenium-bearing wastes that more closely approaches the original TCLP selenium criterion.